Technology update

Sep 20, 2007

Graphene logic gate makes its debut

Graphene-based circuits could become major building blocks for tomorrow's nanoscale circuits and may even provide an alternative to silicon-based electronic devices, which are fast approaching their limits in size and shape. Scientists in China and Canada have now taken an important step forward with the development of a new Z-shaped graphene nanoribbon quantum dot device. The device is the first programmable graphene structure for random access memory (RAM) or logic gate arrays for use in future nanoscale computing.

Recent years have seen a flurry of interest in making graphene-based electronic circuits thanks to measurements of the material's electronic transport properties. Graphene could be ideal for making large-scale integrated quantum dot devices for use in circuits because it can easily be etched using conventional lithography techniques. There is no need for interconnecting wires either, as is the case for devices made out of carbon nanotubes, and continuous graphene sheets can be used.

There are two basic types of graphene nanoribbon: armchair, which is either metallic or semiconducting depending on its width; or zigzag, which is always metallic. The new device made by Jie Chen and colleagues of the University of Alberta and co-workers at the Hefei National Laboratory consists of a Z-shaped graphene nanoribbon junction. In this design, the leads are semi-infinite armchair graphene nanoribbons but the junction is a zigzag graphene nanoribbon. The quantum dot can be trapped when the energy level of the middle junction is altered.

"Intuitively, it is difficult to imagine that a quantum dot can be trapped in metallic zigzag graphene nanoribbons," Chen told nanotechweb.org. "However, we found that this Z-shaped junction device can completely confine electronic states. This finding is very important and significant in making quantum devices out of graphene nanoribbons regardless of whether they are semiconducting or metallic."

The confined states of the quantum dot are possible thanks to the surrounding barriers, which are formed from the interconnection between the armchair and zigzag graphene nanoribbons. What’s more, the confined states exist even when there are considerable irregularities in the edges of the junction, said Chen. "This indicates that the quantum dot device can be made without many constraints and that it is much easier to make than one using heterojunction semiconductors."

The China-Canada team also found that it could control the confinement of quantum dots within crossbar shaped graphene junctions by tuning an external voltage applied to individual junction regions. "Whether a quantum dot exists or not in a cross-junction corresponds to either an 'ON' state (logic 1) or an 'OFF' state (logic 0)," explained Chen. "Compared to conventional CMOS dynamic RAMs, the access time in our proposed design is much faster and the device consumes less power."